Magnetic recording media such as floppy disks have been used for recording data used in computers for a number of years. As software has become more complex, the amount of information to be stored on magnetic storage media has increased dramatically. Floppy disks have previously been used to store this data. However, floppy disks are limited in the amount of information that they can store, and it is desirable to increase the storage capacity or density of floppy disks.
Non-uniform thermal expansion or contraction of the data disk limits the amount of data that can be placed on a floppy disk. Available data disks expand at different rates along different radii of the disk. The disk and its tracks assume an elliptical shape as the environment (such as temperature and humidity) changes, and the read/write head cannot follow the elliptically-shaped on the disk. Thus, tracks in which data is stored have to be wide so that the read/write head sweeps a circular path within a track and locates data despite eccentricities in the shape of the track caused by non-uniform thermal expansion of the disk.
Floppy disks are typically made by coating a polymeric substrate of a polyester such as Mylar.RTM. or polyethylene terephthalate with a magnetic material which records data received from a magnetic recording head. The polymeric material used to make the substrate is typically an oriented polymer that is stretched in one direction so that the long chain molecules of the polymer orient themselves in a particular direction. The long-chain molecules of the oriented polymer become oriented due to the way the substrate is manufactured. A thick sheet of the polymer is stretched longitudinally and is either held in place laterally or stretched laterally (but stretched less laterally than longitudinally) until the thickness of the sheet reaches the desired thickness. Because of this stretching in predominantly one direction, the long-chain polymer molecules orient themselves primarily in the direction in which the polymer sheet was pulled most.
These oriented polymers are sensitive to environmental e.g., temperature, and humidity changes. For example, as the temperature increases or decreases, the polymer expands or shrinks. The oriented polymer expands or shrinks at a different rate in the direction of orientation than it expands or shrinks in a direction perpendicular to the direction of orientation.
Because portions of the substrate expand or shrink at different rates, it is difficult to retrieve data from a particular location. Changes in temperature affect the location of the data, and consequently, a wider data track has to be provided so that the read/write head can locate the data despite the track having an eccentric shape. As a result of this, data cannot be packed too closely on a floppy disk, and lower storage capacities than what can be achieved result. As more storage capacity on a floppy disk is needed, it is increasingly important to provide a substrate that does not suffer the shrinkage and expansion that an oriented polymer such as PET suffers.
Further, it is necessary to limit the amount of expansion with change in temperature. As shown in Table 1, a polyester has a high coefficient of linear thermal expansion. It is desirable to limit the amount of linear expansion so that a data head can accurately locate data on a disk.
It is therefore an object of this invention to provide a high-density data recording medium with essentially uniform expansion and contraction in all directions across the medium, thereby providing it with greater dimensional stability.
It is another object of the invention to provide a floppy disk that has greater data storage capacity than is provided by currently-available floppy disks. Other objects and advantages are apparent from the discussion herein.